1. Field of Invention
The present invention relates to a non-volatile memory and fabricating method thereof. More particularly, the present invention relates to a mask read-only-memory (mask ROM) and fabricating method thereof.
2. Description of Related Art
Most mask read-only-memory (mask ROM) has a structure that includes a plurality of bit lines (BL) and a plurality of word lines (WL) that cross over the bit lines. Underneath the word lines and between two neighboring bit lines, there is a memory cell channel region. For some mask ROM, a data value of xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d is stored according to whether ions are implanted into the channel layer or not. The process of implanting ions into the specified channel regions is often referred to as a code implant process.
The code implant process of a mask ROM includes patterning a photoresist layer on a substrate using a photomask to expose the code implant regions and then conducting an ion implantation to implant ions into the designated regions using the patterned photoresist layer as a mask. However, the photomask that serves as a coding mask in the code implant process may contain single isolated patterns as well as dense patterns. During the transfer of pattern to the photoresist by photo-exposure, intensity of illumination on single isolated patterns will be weaker than intensity of illumination on dense patterns. Hence, deviation in critical dimension between dense pattern regions and isolated pattern regions due to optical proximity effect (OPE) can be considerable. Therefore, the code implant regions in the mask ROM may have non-uniform dimension leading to a misalignment of implanted ions in the channel implant process. Ultimately, wrong data may be programmed into the read-only-memory cells.
To reduce the difference in critical dimension in the mask ROM due to coexistence of dense and isolated pattern regions within the same photomask, technique such as an optical proximity correction (OPC) or a phase shift mask (PSM) is deployed. The optical proximity correction method utilizes a complementary pattern design to eliminate critical dimension deviation caused by proximity effect. However, to deploy the OPC method, a photomask with specialized pattern must be designed. Thus, aside from spending more time to fabricate the photomask, the photomask is more difficult and costly to fabricate. Furthermore, debugging any defects on the photomask pattern after fabrication is very difficult.
In addition, if the coding mask used for coding implant is misaligned or contains some deviation in critical dimension, coding ions originally intended for the designated implant regions may diffuse into the buried bit lines leading to a change in ion concentration. Consequently, there may be a considerable reduction in the current flowing through the buried bit line.
Accordingly, one object of the present invention is to provide a mask read-only-memory (mask ROM) structure and fabricating method thereof that prevents coding ions from diffusing into a buried bit line and leads to a considerable reduction in current flowing through the buried bit line during operation.
A second object of this invention is to provide a mask read-only-memory structure and a fabricating method thereof that prevents the production of non-uniform critical dimensions in a code implant region having both isolated patterns and dense patterns after memory coding as in a conventional code implant process.
A third object of this invention is to provide a mask read-only-memory structure and fabricating method thereof capable of coding the mask ROM with great precision without using optical proximity or phase shift mask technique, thereby lowering cost of production.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a method of fabricating a non-volatile memory. A longitudinal strip of stacked layer is formed over a substrate. The longitudinal strip is a stacked layer comprising a gate dielectric layer, a conductive layer and a cap layer. A buried bit line is formed in the substrate on each side of the longitudinal strip. The longitudinal strip is patterned in a direction perpendicular to the buried bit line to form a plurality of stacked blocks. Thereafter, a dielectric layer is formed over the substrate. The dielectric layer exposes the cap layer of the stacked blocks. A code-masking layer is formed over the dielectric layer. The code-masking layer includes openings for isolated pattern regions and openings for dense pattern regions that expose a portion of the cap layer of the stacked blocks. Due to optical proximity effect, both the isolated pattern region openings and the dense pattern region openings have deviation in some critical dimensions. Using the code-masking layer as an etching mask, the cap layers of some stacked blocks are removed to expose the conductive layer underneath. The cap layer is removed by conducting an anisotropic etching operation. Although the critical dimension of the opening in the code-masking layer is non-uniform, the ultimately formed memory device in this invention is little affected by the non-uniform critical dimension because the anisotropic etching step removes the cap layer of the stacked blocks smoothly and completely. In addition, the cap layer over the stacked blocks need not be completely removed in the anisotropic etching process as long as a portion of the underlying conductive layer of the stacked blocks is exposed for connecting with a subsequently formed word line. The code-masking layer is next removed and then a word line is formed over the dielectric layer so that stacked blocks in the same row are serially connected to form a plurality of coding memory cells. Among these coding memory cells, the ones having electrical connection between the word line and the conductive layer is in a first logic state and the ones having no electrical connection between the word line and the conductive layer is in a second logic state.
This invention also provides a non-volatile memory structure. The non-volatile memory includes a substrate, a buried bit line, a plurality of first stacked blocks, a plurality of second stacked blocks and dielectric layer and a word line. The buried bit line is embedded within the substrate. The first stacked blocks are positioned on the substrate. Each first stack blocks is a stack that includes a cap layer, a conductive layer and a gate dielectric layer. The second stacked blocks are also positioned on the substrate. Each second stacked block is a stack that includes a conductive layer and a gate dielectric layer. The dielectric layer is on the substrate for electrically isolating the first stacked blocks and the second stacked blocks. The word line is positioned over the dielectric layer so that the first stacked blocks and the second stacked blocks along the same row are serially connected together to form a plurality of coding memory cells. Among the coding memory cells, the ones having a first stacked block is in a first data state and the ones having a second stacked block is in a second data state.
The mask ROM coding method according to this invention utilizes the connectivity between the word line and a conductive layer instead of a conventional code implant process to code the memory device. Hence, non-uniform dimensions in the code implant region within a memory device caused by a non-uniformity of critical dimensions between the isolated pattern region and the dense pattern region within a code-masking layer is prevented.
Because the mask ROM coding method according to this invention is not based on the conventional code implant process, the diffusion of coding ions into the buried bit line will not occur. In other words, current reduction within the buried bit line due the diffusion of coding ions no longer is a problem in this invention.
Furthermore, two sets of photomask designs each having a different line/pitch pattern are used to form the stacked blocks. Hence, dimension of the stacked blocks can be reduced to about 0.12 xcexcm. In other words, critical dimension of the memory device can be reduced to about 0.12 xcexcm.
Since the coding process according to this invention does not involve the use of complicated optical proximity correction or phase shift mask technique, cost for producing mask ROM is reduced considerably.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.